1. Field of the Invention
The present invention relates to a radio communication system, a radio station, and a radio communication method.
2. Description of the Related Art
Conventionally, with the radio communication system, technology reducing affects from interference and increasing frequency utilization efficiency has been considered, since the frequency utilization efficiency deteriorates due to a plurality of radio signals interfering with each other. For example, an interference canceller is used for such interference reduction technology. As shown in FIG. 1, an interference canceller 820 in a receiver 800 estimates the propagation path for an interference signal and a desired signal from a received signal, which is received by an antenna 810, and information regarding the desired signal and the interference signal. The received signal includes the desired signal and the interference signal influenced by propagation path functions hi and hd and noise n. The interference canceller 820 removes the interference signal from the received signal by generating a replica of the received signal using the estimated propagation path (e.g., Japanese Patent Application Laid-Open No. 2002-43962).
With the interference canceller 820, a determination unit 821 outputs desired signal symbol sequence candidates and interference signal symbol sequence candidates. A propagation path estimator 822 estimates propagation path estimation values hi′ and hd′. The propagation path estimator 822 multiplies the desired signal symbol sequence candidates and interference signal symbol sequence candidates by the propagation path estimation values hi′ and hd′ so as to generate replicas of the desired signal and the interference signal. The determination unit 821 calculates the difference between the actual received signal and the replica, and outputs the desired signal components of the desired signal symbol sequence candidate and the interference signal symbol sequence candidate, which have a minimum absolute value of the calculated difference, as a received desired signal sequence.
Specifically, the determination unit 821 represents a signal vector obtained by detecting and sampling the received signal on a coordinate as signal points, and determines a symbol sequence corresponding to the received signal replica, which has a signal point constellation closest to the signal vector obtained by sampling, as the received desired signal sequence. In FIG. 1, four signal forms obtained through the QPSK are represented as signal points plotted on a coordinate. Accordingly, a receiver 800 can obtain the desired signal sequence most likely to have been transmitted.
This kind of interference canceller 820 is mostly used in mobile communication. With mobile communication, frequency offset needs to be considered especially on the uplink. For frequency offset compensation, there is a method shown in FIG. 2. A base station 910 transmits a reference frequency fc′ to mobile stations 920 and 930. Each of the mobile stations 920 and 930 receives the reference frequency fc′, and operates an oscillator based on the received reference frequency, obtaining a carrier frequency with little frequency offset between the other mobile station 920 or 930.
Nevertheless, the interference canceller 820 determines the symbol sequence corresponding to the received signal replica, which has a signal point constellation closest to the received signal as the received desired signal sequence. As a result, when the signal points for the replica of received signals corresponding to a plurality of symbol sequences are the same plotted point coordinates, or are extremely close in position, erroneous determination occurs with a high probability.
Furthermore, when the carrier frequency offset between the desired signal and the interference signal is great, the desired signal points or interference signal points, i.e. the signal points for the received signals are observed to quickly rotate on the coordinate, making it difficult to estimate a propagation path by following that quick rotation.
Moreover, in the case of applying such frequency offset compensation as shown in FIG. 2 for base stations, since the base stations are extensively arranged in various places, an additional problem, which some base stations cannot receive the reference frequency from a base station transmitting the reference frequency may develop.
As a result, the effectiveness of the interference canceller is reduced, and the frequency utilization efficiency cannot be appropriately improved.